108 THE STRUCTURE AND LIFE OF BIRDS cnHap. 
(8) The bones of birds that are highly pneumatic 
‘are, relatively to their length, larger in girth than those 
of birds in which the aeration is but slight. 
(g) All young birds have solid bones. As they grow 
to maturity, if pneumaticity is characteristic of the 
species, the marrow dries up and the bronchial mem- 
brane extends into the hollow. 
These facts look tangled and perplexing, but I 
believe it is possible to some extent to unravel them. 
I will begin by considering the case of the diving birds. 
Much aeration of the bones would be an inconvenience 
to them ; as it is, they can regulate the amount of the 
body that appears above the water, sometimes sinking 
till no more than the head is visible. Very often it is 
impossible to see a Red-throated Diver swiniming ina 
mountain tarn. Only his neck stands out above the 
water, and you cannot distinguish it among the reeds. 
The Cormorant uses the same device, but he is not 
equal to the Red-throated Diver in making himself 
heavy or cork-like at pleasure. This power to vary 
their specific gravity resides, no doubt, in the air-sacks, 
which they can at will empty or inflate. Sometimes 
they help diving birds, it is thought, in another way: 
those which lie under the skin about the neck 
and breast of the Gannet may serve as air-cushions to 
break his fall when he dashes into the sea from a 
height of over 100 feet.1. Aerated bones, on the other 
hand, would be a hindrance and not a help to a diver, 
for they would make it harder for him to swim under 
water. Probably, too, the marrow in the bones serves 
1 See on this point a paper by Mr. F. A. Lucas in Matural 
Sctence, January, 1894. 
